1. FIELD OF THE INVENTION
This invention is directed to aqueous coating compositions, methods of preparing these compositions and to the pressure-sensitive coatings obtained on application to a substrate with subsequent removal of water from the composition upon drying. The pressure-sensitive coatings have particular application for copying and recording purposes.
2. DESCRIPTION OF THE PRIOR ART
Pressure-sensitive materials containing microcapsules of liquid colouring materials, or reactants which on contact with surrounding material give a coloured reaction product, the rupture of the capsules creating contrast with the surrounding areas, are known. These encapsulated liquid materials are costly and require care in preparation and in handling.
Pressure-sensitive particles having a dark core and a frangible light-coloured pigment covering are also known. Under pressure the covering breaks and the dark interior becomes exposed. There are limitations in terms of pressure required and resolution and contrast obtained using such materials. Such particles, as disclosed in Canadian Pat. No. 627,609 to Schossberger et al, are limited to light-coloured pigments, for example titanium dioxide.
Other pressure-sensitive coating systems are disclosed by Dalton in his U.S. Pat. Ser. Nos. 3,125,458, 2,313,808 and 2,313,810. These systems relate to aqueous dispersions of milled soap particles and binding agents. The pressure-sensitive coatings are spread on a coloured backing substrate to give a normally opaque pressure-sensitive coating which becomes substantially transparent upon the application of pressure.
According to applicant's U.S. Pat. No. 3,967,034 and copending application Ser. No. 669,754, a pressure-sensitive and heat-sensitive coating was provided having a compactible open porous continuous matrix structure of contiguous fine, discreet, stable, light-scattering, meldable, organic plate-like particles of a solid organic material and fine light-scattering pores. Such particles were selected from fatty acids, fatty alcohols and mixtures thereof or lignin or metal soap particles. The applied coating had a smooth surface and adhered to paper or other suitable substrate materials. The coated material was handleable without marking or marring and showed clear characters when subjected to pressure. However, the composition of this system is produced by a method which necessitates careful control of preparation conditions such as temperature and concentration.
It is an object of the present invention to provide a pressure-sensitive composition which is comparatively easily manufactured and provides a coating which is handleable without marking or marring which is a distinct disadvantage of the Dalton-type systems.
It is a further object of the invention to provide a coating which is compatible with existing aqueous coating equipment.
Further objects and advantages of the invention will be set forth in part in the following specification and in part will be obvious therefrom without being specifically referred to, the same being realized and attained as pointed out in the claims hereof.